I'm a sailor.
Ships sound a certain way, even when not sailing. If they ever sound differently, you know something has happened and you'll have to respond to it. Even if it's just a drill, the response is the same.
My sailing days are actually behind me, but I still get tense when I hear unusual sounds in my house or office.
I never truly appreciated this recurring gag in the Buttons and Mindy segments of Animaniacs until I had a toddler of my own.